Large vertical motor driven pumps are used to pump large amounts of water in almost every power plant that exists. Primary examples are cooling water circulation pumps. They pump water from oceans, rivers, lakes, or cooling towers through the condensers to condense the steam within the boiler and steam turbine flow path. In most cases, the power of the motor-pump ranges from 3000 hp (2200 kw) to 10,000 hp (7500 kw), the shafts and journals range from 7 to 18 inches (175 mm to 450 mm), and the rotational speeds range from 200 rpm up to approximately 600 rpm, though some may rotate up to 1200 rpm. When operating in service in a normal manner, these pumps rotate either clockwise or counter-clockwise, as the designer intends or the application requires. The arrangements addressed in this document consist of a motor situated above the pump with the pump shaft hard-coupled to the bottom of the motor shaft. In almost all cases, the motor has a thrust bearing and an upper radial guide bearing above the motor and a lower guide bearing below the motor. Usually, the pump shaft has one guide bearing near the top of the pump body and an impeller at the bottom of the pump shaft. At the bottom of the pump body there is an axial flow suction bell through which the water enters the pump. For this class and size of motors and pumps, these vertical radial guide bearings are typically bearings with Babbitt bores, 10 to 16 inches (250 to 400 mm) in diameter and 3.5 to 4.0 inches (90 to 100 mm) in axial (vertical) length, and have a diametral clearance between the journal and bearing on the order of 0.001 inch per inch of diameter (0.001 mm per mm of diameter), or approximately 0.010 to 0.016 inches (0.25 to 0.40 mm). Such a clearance may be called a “close clearance”.
In most of these motor-pump arrangements, the thrust bearing and all three guide bearings are lubricated with oil, typically ISO VG 32 or 46 (known as a light viscosity oil or medium viscosity oil) (Note: ISO means International Standards Organization and VG means Viscosity Grade). In certain applications, the guide bearing in the pump is water lubricated using the process water. In this disclosure, oil lubricated bearings are of principal interest. The term “oil” as used herein is to be understood as “a lubricant with sufficient and appropriate viscosity” except where clearly used in a more restrictive sense.
Various methods are used to supply oil to lubricate these bearings. A very reliable method is to have an external motor driven pump, cooler, filter and reservoir package to condition the oil and feed it to each of the bearings and from which to receive drain oil. In this pumping method, known as “pressure fed” lubrication, the external lube oil pump is turned on before the main pump motor, so all of the bearings have lubrication before the motor-pump shaft-line starts to rotate. However, many vertical motor-pump arrangements were built and installed without benefit of an external pumping system as above described, and instead use “pot lubrication” with Babbitt bearings wherein the top of the journal is attached to the shaft and it has an outer wall that extends downward for a distance, perhaps 8 inches (200 mm) leaving a radial gap, approximately 1 inch (25 mm) in radial distance, between the inside surface of this journal wall and the outer surface of the shaft. An inside surface of the stationary pot extends upward in this gap between the journal and shaft. Because there is no seal at the top of the inside surface of the “pot”, it is necessary to keep the surface of the oil a suitable distance below the top of the inside wall of the pot, hence, a compromise is required to set the desired level of the oil along the height of the bearing.
Furthermore, there are two types of oil lubricated bearings typically used in these vertical motors and pumps: The earlier design is a fixed bore bearing, with the bore being circular and having diagonal grooves in the Babbitt surface of the bearing that create a mild pumping action driving the oil from the bottom of the bearing up through these grooves to the top of the bearing, thereby lubricating the bearing. A later design is a tilting pad type as shown in Brady, et. al., published application US 2010/0329890 A1 and U.S. Pat. No. 8,246,313 B2, both of which are hereby incorporated by reference. While the tilting pad design is recognized to be an advantageous design relative to fixed bore bearings, this document addresses fixed bore bearings because there are so many of them in service and retrofit fixed bore bearings using designs based on the invention and improvements shown herein would make significant improvements in their operating performance, thereby making it more difficult to justify retrofitting these motors and pumps with tilting pad bearings or pressure fed lubrication.
As a reconfirmation, the present disclosure is concerned with only fixed bore bearings having a circular bore. A common feature in a very large number of these conventional fixed bore bearings is a series of angled grooves in the surface, approximately 0.18 inches (4.5 mm) wide, 0.12 inches (3 mm) deep, at 45 degrees. Almost all motors are designed to rotate in either direction, and the user may select the rotation required for his application. Regarding pumps, almost all pump impellers are shaped to function effectively in only one direction of rotation, but there are circumstances in which reverse flow through the pump can occur which will cause the pump and the attached motor to rotate in the reverse rotational direction. Consequently, the motor manufacturer and the pump manufacturer design their bearings to function and perform adequately in either direction of rotation.
Circumstances in which a motor and/or pump can rotate in one direction or the other are these: (1) When a motor is installed initially, or when it has been removed, serviced, possibly disassembled and reassembled, and then reinstalled, it is uncertain that the 3-phase electric cables will be connected or reconnected to the motor so as to cause the motor to rotate in the reverse direction upon being started. For this reason, a normal step after completing the electrical connections to the motor but before coupling the motor shaft to the shaft of the pump or other load is to “bump start” the motor to confirm that the motor is properly wired so that the motor rotates in the desired direction when started. Typically, in a “bump start”, the period of time between pressing the start button and then the stop button is so short that the motor does not reach full speed. In this situation, the pump is not connected to the motor so the pump does not rotate at all. (2) When a pump shaft is connected to a motor shaft and the pump discharge piping is connected to a piping system that can remain pressurized after the subject pump stops rotating by means of a pressurized tank or one or more pumps operating in parallel that continue to operate and pressurize the downstream piping system of all of the pumps, and the check valve (also known as a “one-way valve”) in the discharge line from the subject pump fails to close, then the downstream pressure will flow back through the subject pump causing this pump and the connected motor to rotate in the reverse direction. When such a discharge check valve fails to close, then the discharge isolation valve for the subject pump must be closed—either by a power means or manual means—to stop the reverse flow through the pump and to stop the pump and motor from rotating in reverse.
While these bi-rotational circular bore bearings have been in service for decades and are very simple to manufacture and to use, they do have drawbacks. For these vertical radial guide bearings that are typically 10 to 16 inches (250 to 400 mm) in diameter, 3.5 to 4.0 inches (90 to 100 mm) in axial length, the bottom 1.5 inches (37 mm) is below the surface of the oil level in the pot which means that the top 2.0 to 2.5 inches (50 to 63 mm) is above the surface of the oil. When the bearings sit still for a period of time, more than a few days, the oil drains from the journal and bearing surfaces, so that the top 2.0 to 2.5 inches (50 to 63 mm) are dry. On start-up, the motor and pump shafts increase rotational speed rapidly. At the same time, the magnetic forces on the motor rotor tend to pull the motor rotor radially to one side, and because the water flow through the pump almost always flows into a volute with a lateral discharge port, a lateral force component is applied to the impeller. These forces then push/pull the shafts and journals laterally and they are restrained and held in position by reaction forces from the associated bearings. These are illustrative examples of reasons that journal surfaces of vertical motors and pumps can be expected to start in contact with and rub against the respective bearing surfaces.
Being dry, or nearly dry, it is quite common for these guide bearings to have hard dry rubs, leading to Babbitt “wipes” on start-up, including on the first start-up. In this context a “wipe” means that a journal surface has dragged along and rubbed the Babbitt surface with sufficient frictional force to deform or to dislodge an amount of Babbitt and to move it along the surface in the direction of rotation. Sometimes the wipe is merely a “burnish” in which case the surface is only polished, and no other damage occurs. Sometimes the “wipe” is quite severe meaning that there is significant smearing of the Babbitt, generating elevated temperatures in the Babbitt. Occasionally, local temperatures are sufficiently high so as to cause Babbitt to melt and to flow away. Clearly, wiping a Babbitt surface can increase the clearances in the bearing, reducing the ability of the bearing to restrain the motion of the rotor and thereby contributing to high amplitude rotor vibration. These forms of wiping are illustrative of bearing damage that can require a motor-pump to be shut down, sometimes immediately and sometimes at a later time, and can require the bearing(s) to be replaced.
The foregoing leads to these two critical questions: (a) How many rotations of the journals are required before sufficient oil is driven up the angled grooves of a conventional guide bearing to provide adequate lubrication to form a minimum oil film that will permit elasto-hydrodynamic lubrication (lubricated rubbing) instead of dry rubbing? (b) How many rotations of the journals are required before sufficient oil is driven up the angled grooves to provide adequate lubrication to form an oil film that has the “wedge shape” necessary to develop sufficient pressure in the film and sufficient film thickness to keep the journal separated from the bearing, thereby avoiding or stopping rubbing?
To begin to develop a solution to this problem it is very helpful to develop an understanding of the problem. Consequently, a test arrangement duplicating a pot oiled journal bearing was fabricated and initial testing of an existing conventional bearing with alternating diagonal grooves was performed at the normal operating speed of a particular motor-pump, approximately 500 rpm. To see the oil flow patterns within the bearing, the top lip of Babbitt above the circumferential drain groove was machined away. Upon starting the test journal to rotate in the test bearing, the problem was immediately visible to an observer: It took approximately two (2) rotations for the oil to come up one set of angled up-flowing grooves into the circumferential groove, but then the oil immediately went down the other set of grooves with opposite angles, or down-flowing grooves. There was a very small amount of oil circulating in the circumferential oil groove at the top of the bearing, and almost no oil went out through the radial drain oil passageways to the outside to be cooled. Only a limited amount of oil appeared to travel up and down the grooves. Further, one could conclude that when this oil flow reached the bottom of the down-flowing grooves, it continued to flow downward as a jet effectively pushing fresh cooler oil away so that only a small amount of fresh cooler oil would enter the up-flowing grooves.
While the owner/operators are aware of the risks of operating vertical motor-pumps with bearings of this design and are aware that an external pressure-fed pumping system, if implemented, would resolve the bearing damage and rotor vibration problems, they are quite reluctant to change the means of lubrication to an external pump for a number of reasons, including (a) the difficulty to redesign the motor and pump structures to accommodate the required oil conditioning equipment, (b) the cost of the added retrofit equipment, and (c) especially, the cost of the lost production time (measured in hours) necessary to implement the changes.
An object of the present invention is to provide improvements to the design of the existing fixed bore bearings that, for journal bearings that are starting “dry”, will provide a substantial reduction of the time period for oil to be driven to the top of the bearing thereby advancing the start-time for elasto-hydro-dynamic lubrication (wet-rubbing) as well as advancing the start-time for a full film of oil to exist in the journal bearing sufficient to permit a proper wedge film shape to develop with pressure and thickness necessary to keep the journals separated from the bearings. It is anticipated that the improvements described herein will provide a significant improvement in the performance of these vertical radial guide bearings during the start-up portion of motor-pump operation in normal rotation, and will function adequately well to provide oil films with adequate pressure and thickness to develop during the few short periods of time that reverse rotation occurs. Initial tests of designs incorporating the invention have shown that these improvements are attained.
The improvements to the designs of conventional fixed bore bearings are based on knowledge of which rotational direction is normally used, and on implementation of selected principles of lubrication theory and practice. One of the fundamental principles of lubrication that is germane to the improvements advanced herein is that the film pressure in and adjacent to any groove has no pressure, that is, 0 psig, and therefore, a film with zero pressure cannot keep the journal from contacting the bearing. The implication is that there should be large areas of Babbitt surface without any grooving so as to be able to develop fully developed wedge shaped oil films that are able to develop sufficient pressures and thicknesses to keep the journal separated from the bearing, recognizing that a compromise must be made to provide an adequate number of grooves of sufficient size to be able to supply adequate new cooled oil to the film.